Many people have flat roof extensions. Seems that they were all the rage some time ago. We were obviously hardier then as well given that most are poorly insulated, draughty and can be cold and quite miserable places. This is commonly re-enforced by the fact that they were often designed to be bathrooms.
Bathing in a room that is exposed on two or three sides with a cold roof is not particularly pleasant in the winter unless one has a suitable radiator on full and the extractor fan on a humidity control.
These flat roof extensions are therefore having to do a lot, especially with regards to moisture. Cold walls, windows and roofs with high levels of humidity just leads to one thing. Condensation and mould.
Most of the heat in the room will be lost through the roof (assuming that it has minimal insulation). So the most obvious job is to get the roof well insulated.
When looking at this the most obvious solution seems to be to install the insulation to the underside of the ceiling. However this can lead to problems. If you have insulation below the ceiling then any warm and very moist air that manages to get above this will condense on the even colder underside of the felt / top covering of the roof. This will then condense and fall back on top of the insulation. This can cause a lot of problems with electrics, damp, rot etc. So this solution is not recommended.
So unfortunately the roof has to be taken apart somehow to either install the insulation just below the main roof covering, or above it. Putting the insulation on top seems to be a easy option, but there is the issue of air circulation, so just plonking some rigid waterproof insulation on top doesn’t work as there is an air flow through the existing roof structure that needs to be altered as well otherwise the external cold air will just flow under your lovely new insulation and effectively make it useless.
So the only real way of tackling this is to look at seriously upgrading the roof when replacing the roof covering. This allows you to change around the air circulation so that there is no external air source into the structure. This then allows you to insulate next to the underside of the roof covering. Airtight and water tight membranes are important here though. Your roof covering does not breathe and so you must stop any warm moist air from reaching it. By using membranes and also by adhering the insulation to the roof covering you effectively create a warm surface in the room, thus stopping condensation from forming on it.
All in all, it might be worth thinking about replacing the flat roof with a well insulated sloping roof!
To make all of this clearer follow this link to Brian Murphy’s excellent GreenSpec website – he has pictures that speak a lot of words!